"We are not just declaring the necessity of the [Syrian] opposition's acceptance of the Russian-U.S. conference initiative. We are working with the opposition and have been visited by leaders of practically all opposition groups except for the National Coalition, which has our invitation," Lavrov told reporters in Moscow on Monday.

"Moscow is always ready to receive the leaders of the Syrian National Coalition," he said.

"Everyone should be spoken to, and involvement, rather than isolation, is the key to success in any conflict situation," Lavrov said.

"I am confident that results will be achieved if all the participants in the Syrian resolution, those outside of Syria, launch a policy of involvement with all Syrian groups, including the government and every opposition faction," the minister said.

Exclusion of any party from the negotiations will make it impossible "to come out of the standoff, which has been dragging on for over 18 months," Lavrov said.